Aerospace Fasteners Mix It Up

Overall trends calling for less weight are posing new challenges for the fasteners used in airplanes, Mars rovers, and lunar landers. Whether fasteners connect carbon composites and metals in wing structures or join landing gear components, they must reliably keep together millions of parts in a single craft, subject to huge loads and stresses.

Although industrial fasteners have to be strong enough to hold up structures as big as San Francisco's Golden Gate Bridge, a high-strength Grade 8 industrial bolt is still only about 80 percent as strong as a high-strength aerospace fastener. After all, the craft using those fasteners have to rove around on Mars, or maybe just carry passengers from London to Paris.

An A380 wing panel is being assembled on this automated line using Electroimpact E4380 automatic drilling and insertion systems for permanent fastener installation. The machines clamp the skin and stringer together, drill holes, insert and form aluminum rivets. They also drill and coldwork holes and then insert titanium pins or lockbolts.

In all aerospace materials, including fasteners, the requirement for high strength-to-weight ratios is as important as the requirement for high absolute strength, Bob Gurrola, manager of custom applications engineering for Alcoa Fastening Systems, said in an interview. Fasteners also must be as light as possible, whether they're in the cockpit holding together the armrest of the pilot's chair, or keeping the wings together.

Because of these requirements, the dominant material in aerospace fasteners for the last 30 years or so has been titanium. But in the high-temperature environments on aircraft, such as areas near or in the engine, temperatures can reach 1,100F or higher. There, titanium doesn't work as well, so multi-phase materials or nickel-based materials may be used.

Fasteners for spacecraft also must be lightweight, but they have additional requirements. The lubricant used in typical aerospace fasteners can be an issue in a zero-atmosphere environment, where they tend to vaporize. If that occurs near sensitive instrumentation, some lubricant might get on a lens and obscure it. Most spacecraft fasteners must operate without a lubricant or with one that's space-compatible.

"Another big difference, and it's critical, is that fasteners used in propulsion or fueling systems have to be compatible with that specific environment," Gurrola told us. Materials used near liquid oxygen tanks on some space vehicles can't react with that oxygen; they have to be inert. Fasteners used in space must also be reliable in design, materials, and manufacturing process; operate in extremes of temperature; and do all this for years.

Although the dominant material for commercial aerospace components has been aluminum, that pattern is changing. The entry of carbon composites into aerospace manufacturing has been revolutionary, said Gurrola, and that's true not only for the material itself. "It also required a whole new way of approaching the problem of how you fasten these components," he said. "Early on, engineers designing structures with composite treated it like 'black aluminum,' instead of taking advantage of the differences. Similarly, some fastener manufacturers also did not consider these differences. But designing structures with composites, and fasteners for them, requires a totally different approach."

The lightning strike issue isn't about frequency so much as it is about catastrophic results. If you've only got a (for example) 1% chance of something happening, but that something has catastrophic results--people dying, lawsuits--then that's something you've got to protect against, or at least not encourage, in your materials and assembly process selection.

I never thought that lightening strikes on aircraft was so common. I read that it happens 2 times per year on average, per airplane. I have seen electrical discharge responsible for fastener loosening and in some cases, ejecting.

There is a downside to composite pieces, price. Bolting parts together will always be around. I designed a mechanical system that ended up having over 60 bolts.. it was cheaper than with none, that was for sure.

Glad you liked the article. The whole issue of the grounding of composites used in aircraft has been widely misunderstood, so I thought it was a good idea to include some clear discussion on that issue. Could you clarify your question about comparisons between fasteners for composites and fasteners for metal? What sort of comparisons do you have in mind?

Excellent post Ann. I know the longevity of any fastener is dependent upon the application and use. Relative to composite fasteners, do we know how they "stack up" relative to metal fasteners? I have seen no data that tries to correlate life cycles of either type. Great point also about the grounding of composites. I know this must be a huge issue but not talked about too much in the literature.

That's true of course. The question is, given an increase in composite use, whether fasteners will be used in high enough quantities in repair to make up for the lower overall quantities in manufacturing.

Many of the new adhesives we're featuring in this slideshow are for use in automotive and other transportation applications. The rest of these new products are for a wide variety of applications including aviation, aerospace, electrical motors, electronics, industrial, and semiconductors.

A Columbia University team working on molecular-scale nano-robots with moving parts has run into wear-and-tear issues. They've become the first team to observe in detail and quantify this process, and are devising coping strategies by observing how living cells prevent aging.

Many of the new materials on display at MD&M West were developed to be strong, tough replacements for metal parts in different kinds of medical equipment: IV poles, connectors for medical devices, medical device trays, and torque-applying instruments for orthopedic surgery. Others are made for close contact with patients.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.